1. Field of the Invention
The present invention relates to an ink jet recording apparatus, and more particularly to an ink jet recording apparatus for recording onto a recording medium by discharging the ink through discharge ports onto the recording medium by use of the heat energy generated by block driving a plurality of electricity-heat converters.
2. Related Background Art Information
Among various recording methods currently known, the so-called ink jet recording method, which is a non-impact recording method that generates almost no noise during recording and permits printing at high speed onto plain paper without requiring special fixing processing, has been appreciated as a quite effective recording method. Such an ink jet recording method is one in which the recording is performed by discharging fine droplets of recording liquid called ink to apply them onto a recording medium such as paper.
This ink jet recording method is such a method that in recording, electricity-heat converters provided in ink channels (hereafter referred to as nozzles) communicating to fine discharge ports for discharging the ink are energized and heated, thereby heating the ink around heat generating portions (hereafter referred to as heaters) of the electricity-heat converters, and the ink is discharged through the discharge ports by use of the pressure caused by abrupt changes of volumes produced by bubbling. One of the driving methods for an ink jet recording apparatus is a so-called division driving method in which heaters are divided into n groups each for plural bits of a driving signal, which are driven in sequence. The reason is that a current of 250 mA is required to drive one heater normally. Therefore, for example, when 64 nozzles are concurrently driven, a current of 16A will be required, so that a large power supply is needed, or heavy wirings are required to carry large currents. On the contrary, for example, if energization is made each for 16 nozzles in four times, the current required concurrently will be largely reduced up to 4A. Moreover, if energization is made each for eight nozzles in eight times, it can be reduced to 2A.
However, in recording with such a driving method, there is a problem that positions of liquid droplets impinging on a recording medium are different between groups, so that the image quality may be degraded.
It has been found that such impact position shifts between groups are caused by following two factors, as a result of having observed minutely this phenomenon of impact position shifts. The first factor is necessarily caused by the division driving method. That is, it is caused by differences between energization timings for groups, and the relative movement between the recording medium and recording head. The second factor is due to the fact that when a plurality of nozzles are driven in division for discharging substantially at the same time within each group, and at fixed intervals between groups, the discharging speed for the first group being driven first is faster than those for the second and following groups which are sequentially driven.
Next, a specific example of the first factor in recording with a recording head and a recording apparatus to which the above-described conventional division driving is applied will be described. When a head having 64 nozzles is driven at 360 DPI, 6.3 KHz, a pulse width of 3 .mu.s, four division driving pause time of 0.5 .mu.s (see FIG. 7), and a clearance between head and recording medium is 1.2 mm, the impact position shift owing to differences between energization timings for groups which is the first factor as above described is, ##EQU1## That is, a shift of 0.07 dots occurs on recording medium.
The impact position shift Ws owing to differences between discharging speeds for groups, which is the second factor as above described, is shown in Table 1 as given below from experiments of the present inventors.
TABLE 1 ______________________________________ Average Reaching time Shift from discharge to recording dot in first speed medium group Group (m/s) (.mu.s) (dots) ______________________________________ 1 12 100 -- 2 9 133 0.23 3 8 150 0.36 4 8 150 0.38 (See FIG. 9) ______________________________________
As above described, the impact position shift due to differences between energization timings for groups is small, such as 0.07 dots, and in almost inconspicuous area, but the impact position shift due to differences between discharging speeds for groups may correspond to a maximum of 0.38 dots, having bad effects on the print quality.